Found this info on trailer tires at Discount Tire:
Speed
•All "ST" tires have a maximum speed rating of 65 mph.
•As heat builds up, the tire's structure starts to disintegrate and weaken.
•The load carrying capacity gradually decreases as the heat and stresses generated by higher speed increases.
Time
•Time and the elements weaken a trailer tire.
•In approximately three years, roughly one-third of the tire's strength is gone.
•Three to five years is the projected life of a normal trailer tire.
•It is suggested that trailer tires be replaced after three to four years of service regardless of tread depth or tire appearance.
Mileage
•Trailer tires are not designed to wear out.
•The life of a trailer tire is limited by time and duty cycles.
•The mileage expectation of a trailer tire is 5,000 to 12,000 miles.
Here's the entire link with some interesting facts:
http://www.discounttire.com/dtcs/inf...rTireFacts.dos